1. Field of the Invention
The present invention relates to semiconductor integrated circuit devices with a function of measuring temperatures, and more particularly, to a technique suitable for the temperature-to-voltage converter mounted in a semiconductor integrated circuit device.
2. Description of Related Arts
The so-called contactless IC card with a semiconductor integrated circuit device and an antenna mounted within the card implements various functions such as exchanging information between an interrogator and the semiconductor integrated circuit device, transmitting the data retained in the contactless IC card, and holding the data that was transmitted from the interrogator.
The semiconductor integrated circuit device mounted in the contactless IC card receives high-frequency signals from the interrogator by means of the antenna mounted in the contactless IC card, rectifies and smoothens the voltages developed at both ends of the antenna, and forms the internal voltages required for the operation of internal circuit.
In recent years, there has been a growing demand for more advanced functionality of the contactless IC card. Coupled with this tendency, increases in the current consumption of the semiconductor integrated circuit device mounted within the contactless IC card are making sufficient internal voltages difficult to generate. To extend the communications distance with respect to an interrogator, the circuits mounted in the contactless IC card needs to be composed so as to operate at lower voltages.
Traditionally, the ambient temperature of a semiconductor integrated circuit device has been measured by conducting a temperature-to-voltage conversion based on the temperature characteristics of diodes or MOS transistors, and then further converting the voltage into an easily processable digital signal by means of an A/D conversion circuit or the like. These methods are disclosed in, for example, Japanese Patent Laid-open No. 2000-307111 (hereinafter, referred to as Patent Reference 1) and Japanese Patent Laid-Open No. 2003-258111 (hereinafter, referred to as Patent Reference 2).
Patent Reference 1 describes a semiconductor integrated circuit device that allows the detection and control of its ambient temperature to be conducted with a one IC chip. Also, Patent Reference 2 describes a temperature detection circuit that can, by operating only when it judges temperatures, reduce current consumption, thus reduce increases in IC chip temperature with an operating current, and conduct accurate temperature judgments.
To augment the temperature dependence of the voltage to be used to detect temperatures in the means shown in Patent References 1 and 2, however, there has been a need to connect a plurality of diodes or MOS transistors in series. This has posed the problem in that the corresponding circuit encounters difficulty with its operation under a low power supply voltage due to increases in operating power supply voltage.